USC coach Lane Kiffin is seeking one attribute in whomever he chooses to start at left tackle against Cal on Saturday:

Effort from start to finish.

Kiffin wasn't happy with the effort of starter Aundrey Walker in the second half of last week's loss at Stanford and opened his spot up for competition. Walker and freshman Max Tuerk (Santa Margarita High) shared first-team reps during practice Tuesday.

“We're looking for finish,” Kiffin said. “Guys are going to get beat. Guys are going to drop balls. You're going to make mistakes. But we won't tolerate lack of great effort, not when you put on these jerseys that all these great players have worn before. I think Aundrey's going to respond well to this.”

Before the season started, I asked center Khaled Holmes whether USC's offensive line could be better in 2012 than in 2011 despite the loss of All-America left tackle Matt Kalil.

"It's going to be hard to replace Matt," Holmes said. "He's a once-in-a-long-time type of player. He's a very special talent. It's going to be difficult. But we do have four returners. And Aundrey (Walker) has looked really good at that spot. So we'll be all right."

The line, sans first-round pick Kalil, had a "great" training camp, according to USC coach Lane Kiffin. Then the season started, and the unit was just OK in the opener against Hawaii.

"Inconsistent," Kiffin said. "Matt (Barkley) saved us from a couple sacks. I know after meeting with them (Sunday) that they're ready to play better."

This week's portion of our USC training camp preview concludes with a look at the team's three biggest concerns. We'll resume the preview Monday, leading into our late-night coverage of the 2012 Trojans' first camp practice.

1. Defensive line

USC lost three starters from last season's line, including first-round draft pick Nick Perry. Then a part-time starter, senior end Devon Kennard, suffered a possible season-ending torn pectoral muscle in late July. That left only two linemen, senior end Wes Horton and redshirt-sophomore tackle George Uko, with any substantial starting experience. But Lane Kiffin and his staff weren't unprepared for this. They signed six defensive linemen in 2011 and three more this year. Many of them will play; how effectively remains to be seen.

Continuing our countdown of the 50 most valuable USC football players of 2012:

NO. 21 -- OG MARCUS MARTIN

Vitals: 6-3, 325, sophomore

Comment: Heading into Week 3 last season, Lane Kiffin was looking for someone -- anyone -- to step up, seize and own the left guard position. Martin was USC's third starter in as many games. He never surrendered the job. Not bad for a freshman rated as a three-star recruit coming out of Crenshaw High. Now Martin goes from unknown to known as an interior anchor for a line that returns four starters. USC has plenty of young talent at his position, but Martin is the only one with 10 games of starting experience. Many of USC's longest runs went to the left side last season, and that trend continued in spring. The identity of the left tackle has changed, from Matt Kalil to Aundrey Walker. The left guard remains the same. It's Martin, suddenly a valuable member of a team with championship aspirations.

(About the rankings: The criteria used to derive the rankings included players' past and projected production; their general importance to the team; how indispensable they are according to the depth chart; their position versatility; their ability to play special teams; and their leadership skills and other intangible traits.)

We have to give defense and special teams some love in our USC over/under series, so here goes:

Aside from takeaways, no defensive statistic is more important than points allowed. The Trojans ranked a very respectable 45th nationally in points allowed last season at 23.58 per game. The defense should be even better this season. Will the points-against stat reflect that?

Presenting the seventh lesson learned during USC spring football practice:

No. 7 -- Sophomore Aundrey Walker is the future -- and present -- at left tackle.

Through the first week of spring camp, the competition to replace NFL-boundMatt Kalil at left tackle wasn't much of a competition at all. Kevin Graf played on the left side, Walker played on the right, and Lane Kiffin said the following: “If they're both playing really well where they're at, there'd be no reason to move them.”

Over the final four weeks of spring camp, the competition to replace Kalil wasn't much of a competition at all either. Except it was Walker on the left and Graf on the right, and Kiffin said this: “He (Walker) just fits better over there (with) his length and his size. When you look at prototypical left tackles, that's what they're built like."